Conversion of analog images into digital data has become widespread for a variety of applications, including storing, manipulating, transmitting and displaying or printing copies of the images. For example, images captured in photographic media are converted to digital data and stored on compact discs for readout and display as a video image, as exemplified by the KODAK.RTM. Photo-CD system, or reproduced employing various types of color printers. In order to convert the photographic image into an image frame set of digital line data, the film image frame is transported through a film scanning station past, and illuminated in each scan line with a linear light beam of uniform, diffuse illumination, typically produced by a light integrating cavity or integrator.
The light transmitted through the illuminated scan line of the image frame is typically focused by a lens system onto a linear CCD array, image detector which typically produces three primary color light intensity signals for each image pixel that are digitized and stored. The digitized signal values for each scan line may be formatted to a standard for video recording and display and stored on compact disc or magnetic media or reproduced by a color laser printer or the like. Such film scanners take a variety of forms, and the various common aspects of film image frame digitizing, particularly line illumination and linear CCD array-based digitizers, are described in greater detail in commonly assigned U.S. Pat. No. 5,155,596.
In order to perform line scanning of an image frame of photographic negative filmstrips, it is necessary to provide an accurate film transport mechanism to transport a filmstrip into a scanning gate and hold the image frame flat in alignment with a scanning aperture. Typically, the linear CCD array and scanning light beam are stationary, so that the light beam illuminates a line of the filmstrip image frame, and a line of digitized data is stored. The scanning gate containing the filmstrip image frame is incrementally moved or translated line-by-line by a stepping drive motor until the entire image frame is digitized. Then a new image frame is positioned and flattened for scanning and digitizing. Such a scanning and digitizing system for Photo-CD conversion is embodied in the KODAK.RTM. PIW" Model 2400 Photo-CD scanner system marketed by the assignee of this application.
In this film scanner, the scanning plane is vertical and the stationary scanner components are oriented horizontally. An operator introduces the negative filmstrip from the left side into a slot leading to a film track and drive mechanism to position an image frame in a filmstrip scanning gate and the image frame is clamped. A belt driven roller assembly advances the film strip through the filmstrip scanning gate past the stationary scanner components in a first pass or "prescan" operation for scanning the image frames at a low resolution sufficient to provide a video display of the image frame on a monitor for viewing by the operator.
The line pre-scan data outputted from the linear CCD array for each image frame is digitized and sent to an external computer which determines image frame boundary line scan numbers and analyzes the scan line data to derive image density and color balance correction factors. The correction factors and boundary line scan numbers are transmitted back to the scanner computer for use in the high resolution "main-scan".
The scanned and digitized image pixel pre-scan line data is also displayed on a monitor for viewing by the operator. The operator may further adjust the displayed color balance or tone and intensity of the color display while viewing the result of the adjustments until satisfied, whereupon the adjustment factors for that image frame are stored. The orientation of the image may also be stored with the digitized data so that the CD player can rotate the image data 90.degree. for display as a video image at the same aspect that the image was captured by the photographer.
As each image frame is scanned in this first pass, the scanned image frames of the vertically oriented filmstrip are advanced into a stationary take-up chamber. The take-up chamber is provided within the scanner to temporarily hold the filmstrip and isolate it from other apparatus that it could catch on and to keep it clean.
After all image frames are scanned, the trailing end of the filmstrip is advanced in the reverse direction into the filmstrip scanning gate one frame at a time. The filmstrip image frame is clamped and translation stage translates the image frame to the start scan position, and then translates each image frame through the scanning station. The image frame is scanned at high resolution for digitizing the image as a field of data associated to the data derived in the low resolution scan of the same image frame. As the next image frame is advanced into the scanning gate, the filmstrip is transported back out the same slot that it was slipped into for removal by the operator when scanning of all frames is completed. Thus, positioning of the next filmstrip to be scanned must await the complete ejection of the filmstrip being scanned.
In the Model 2400 Photo-CD scanner, a single, quadrature-type, perf sensor is utilized to derive coarse position data. Translation drive motor steps of 0.002 inches (0.0004 cm) per half step are used to interpolate between detected perfs to derive fine position data. The quadrature-type perf sensor is constructed of a pair of optical edge detectors arranged parallel to the direction of motion of the filmstrip in the path occupied by the film perfs. The two detectors are spaced apart a distance that corresponds to a multiple of the leading edge to leading edge distance between successive perfs. The resulting sense signals of each detector are 90.degree. out of phase, unless there is a defect in a perf, e.g. a torn edge or an edge partially or totally obscured by a splice tape or jerking and slippage of the filmstrip in the belt or roller drive.
FIG. 1 shows in a simplified block diagram the layout of the quadrature-type perf sensor 10 located to the left of a scanning aperture 12 in alignment with a film transport path 14. Positioning of the sensor 10 takes into account the ISO standard 35 mm sprocket hole number of eight (8) holes per image frame length (38 mm), the sprocket hole spacing of 4.75 mm and the sprocket hole width of 2.75 mm. The sensor 10 includes the sprocket hole edge detectors 11 and 13 spaced apart by 10.50 mm to effect the 90.degree. out of phase detection of each sprocket hole edge and provide a pair of square wave sense signals that are 90.degree. out of phase.
A drive belt 16 engages the film strip edge (not shown) at the nip and transports it to the right during low-resolution pre-scan under control of motor 19. After each image frame is pre-scanned and advanced into the take-up chamber, the trailing edge of the filmstrip image frame is to the right of the scanning aperture 12.
As the film image frames are pre-scanned, the perf sensor 10 provides the perf edge sense signals to the scanner computer 17. The scanner computer 17 maintains a count of the perf signals and initiates the recording of pre-scan line numbers from each perf edge detect by interpolating the line numbers from stepper drive motor command signals. A perf number vs. pre-scan line number table is generated in the scanner computer. Pre-scan image data is then analyzed by the external computer's frame line detection algorithm to identify frame boundaries. The external computer then passes each frame boundary location to the film scanner computer in terms of pre-scan line number.
The scanner computer 17 also determines the number of the last detected perf where the trailing end of the filmstrip advances to the right of (and results in no output from) the perf sensor 10. The scanner computer 17 provides and counts the stepper motor half step pulse signals and generates a "pseudo-perf" output signal at a multiple count thereof from the preceding detected perf or generated pseudo perf output signal. The pseudo perf output signals are counted as the stepper motor 19 drives the last image frame of the negative filmstrip through the scanning aperture 12. The stepper motor 19 is halted on reaching the set count of 24 pseudo perfs, leaving the filmstrip trailing end just right of the scanning aperture 12. The distance and sprocket hole number depends on the distance from the perf sensor 10 to the center point of the scanning aperture 12 and the nominal rest position of the trailing end under the right end of the drive belt 16. Again, a pseudo perf number vs. pre-scan line number table is stored for the last image frame.
The generation of the pseudo perf signals properly distanced in line scan number from the last detected filmstrip perf is difficult in a number of situations. During processing and the initial making of a set of prints, full length film rolls are spliced end to end. After printing, the film rolls of each customer order are severed at image frame borders into filmstrips of four or five image frames to facilitate packaging with the prints. The severed ends of the filmstrips vary considerably in the location of the first intact sprocket hole from the severed edge.
FIGS. 2 and 3 show examples of the variability encountered in the distance from the severed edges 18 to the nearest sprocket hole 22 at the trailing ends of such filmstrips 20. Also, one or two of the sprocket holes 22 at the edge may be entirely or partly covered with splicing tape 24 as shown in FIGS. 4-7. In FIGS. 2 and 3, the pseudo-perf signal should be generated at the correct time after detecting the last sprocket hole 22. In all other cases, the pseudo perf signal could be generated too late, throwing off the entire set of perf number vs. pre-scan line number tables for the filmstrip during the main-scan.
After all of the image frames are pre-scanned, the drive belt 16 is reversed in direction to drive the filmstrip to the left and used to advance each image frame into the scanning aperture 12 to perform the high-resolution main scan. The control unit utilizes the pre-recorded perf vs. pre-scan line number tables for each image frame to identify the perf closest to the right image frame edge. The drive belt 16 advances the filmstrip to the identified perf, and then the stepping drive motor steps are used to interpolate between perfs to arrive at the frame edge. The filmstrip image frame is clamped in the filmstrip scanning gate to commence the main-scanning as the filmstrip scanning gate is translated in the scanning station.
Problems to be Solved by the Invention
The usage of a single perf sensor on one side of the scanning aperture results in an open loop control system for at least the last three image frames on the negative filmstrip. Moreover, for a negative filmstrip four image frames long, the last three image frames are pre-scanned with the trailing edge beyond the perf sensor. Film slippage and jams that occur during the pre-scan of the second through fourth image frames may be confused with torn perfs and not detectable until the filmstrip has (or should have) arrived back at the perf sensor for main-scan. This causes difficulties for the system if, as the fourth image is aligned for main-scan, a slip occurs. The first three image frame main-scans must be discarded as the image frame alignment is in question.
Another problem that results from using a single perf sensor is the inability to differentiate film slippage from perf defects (i.e.; torn perfs). In addition, the many trailing end, film edge configurations, as shown in FIGS. 2-7, introduce inaccuracy in the transition from real to pseudo perf counts.